The present invention relates to a zoom lens assembly, and more particularly to a mechanism for taking up backlash or lost motion along an optical axis between magnification-varying lens groups that are axially movably supported in a cam ring of a zoom lens barrel.
Some zoom lens assemblies comprise at least front and rear magnification-varying lens groups axially movably supported in a zoom lens barrel. The rear lens group has a cam follower slidably placed in a cam groove defined in a cam ring that is rotatable about its own axis in the zoom lens barrel. When the cam ring is rotated, the rear lens group is moved along the optical axis of the zoom lens barrel under the control of the cam follower that slides in the cam groove in the cam ring. The front lens group has either a cam follower guided by another cam groove defined in the cam ring or has a helicoid gear meshing with a helicoid gear on the cam ring.
The front and rear lens groups are subject to axial backlash or lost motion with respect to the cam ring because there is always an axial clearance between the cam followers and the cam grooves and also between the helicoid gears no matter how accurate the cam followers, the cam grooves, and the helicoid gears are machined and assembled.
The axial backlash or lost motion in zoom lenses is undesirable as it tends to adversely affect zooming movement of the zoom lens assemblies. For example, because of the axial backlash, an intermediate zoom setting reached when the zoom lens assembly zooms out from the tale end of the zooming range differs from an intermediate zoom setting reached when the zoom lens assembly zooms in from the wide end of the zooming range.